Water oxidation catalyzed by polyoxometalate containing iron cubane structure under visible light

Hydrogen is a renewable and clean energy source that can be produced using photocatalytic water splitting, a promising technique to address environmental challenges. Photocatalytic water splitting involves two half-reactions: hydrogen evolution and oxygen evolution. The water oxidation reaction is the rate-limiting step due to its complex four-electron, four-proton transfer process. In this study, three iron-containing polyoxometalate (POM) catalysts—Na₂₁[NaFe₁₅(OH)₁₂(PO₄)₄(A-α-SiW₉O₃₄)₄]·85H₂O (Fe₁₅POM), Cs_4.5K₁₄Na_1.5[Na₂Fe₁₄(OH)₁₂(PO₄)₄(A-α-SiW₉O₃₄)₄]·105H₂O·2Na₃PO₄ (Fe₁₄POM), and K₅[SiFe(H₂O)W₁₁O₃₉] (FeSiW₁₁)—were synthesized. Among them, Fe₁₅POM incorporates an Fe₄O₄ cubane structure. The photocatalytic water oxidation activities of these catalysts were investigated in a dye-sensitized system comprising [Ru(bpy)₃]²⁺ as the photosensitizer and S₂O₈²⁻ as the sacrificial electron acceptor. Reaction conditions were optimized by varying the concentrations of the photosensitizer, sacrificial reagent, and POMs. In addition, the effects of pH, reaction process, and catalyst stability were evaluated using a range of characterization techniques. Fe₁₅POM exhibited the highest activity among the tested catalysts, generating 5.40 μmol of O₂ with a yield of 14.4%. The cubane structure not only improves the stability of the iron-based POM but also facilitates efficient charge transfer during water oxidation. This study offers a new direction for the development of iron-based cubane POMs for efficient water oxidation catalysis.